In association with supply and consumption of energy represented by an electric power plant, a diesel engine, and a boiler, nitrogen oxides such as nitrogen monoxide (NO) and nitrogen dioxide (NO2) are discharged.
Nitrogen oxides discharged into environment cause photo-chemical smog and the like, and a countermeasure for this problem is discussed as an important subject of environmental problems in large cities, and also attracts attention as a cause of global warming that is particularly problematic in recent years.
As a method of reducing nitrogen oxides, a combustion system, a catalyst system, a selective catalyst reducing system (SCR), an ammonia spraying system and the like are known, and in recent years, there are known a method of reducing nitrogen oxides by combining the aforementioned catalyst system with techniques such as nonthermal plasma and electron beams, and a method of reducing nitrogen oxides by combining other plasma and electron beam system with a method using chemicals such as ammonia, hydrogen peroxide and calcium chloride, catalysts and the like.
Among such methods, a plasma-chemical hybrid method attracts attention. This method is a method of cleaning exhaust gas containing nitrogen oxide, including: supplying air to a discharge plasma reactor to generate of radical gas; supplying the resultant radical gas to an oxidation reaction area, while supplying the exhaust gas from a line different from the radical gas generation line to the oxidation reaction area, to oxidize nitrogen oxide in the exhaust gas to oxidized gas containing NO2 by the radical gas; and then bringing the oxidized gas into contact with a reducing agent aqueous solution containing a compound such as Na2SO3, Na2S and Na2S2O3 in a reduction reaction area to reduce NO2 to nitrogen gas, thereby achieving the cleaning (see for example, Patent Documents 1 to 4).
For bringing the plasma-chemical hybrid method into practice, it is necessary to continuously replenish a chemical solution to a chemical scrubber for keeping the ability of removing nitrogen oxide even in a continuous processing condition. For example, a method of supplying an additional reducing agent aqueous solution and an additional alkaline aqueous solution to a circulating processing liquid just before introducing into the reduction reaction area while keeping the pH at 11, and controlling the oxidation-reduction potential (ORP) to −50 to −250 mV is proposed (Non-patent Document 1).
The ORP is an index indicating whether the aqueous solution is in an oxidative atmosphere or a reductive atmosphere, and the lower the value of the ORP (0 mV or less), the stronger the reductive atmosphere is, and the atmosphere shifts from reductive to oxidative as the value increases, and a reductive reaction hardly occurs at an ORP of 100 mV or higher.
In an acidic atmosphere of pH 6 or less, Na2SO3 serving as a reducing agent reacts with an acid and thus be wasted, and harmful SO2 is generated, so that it is necessary to keep the pH at 6 or higher.    Patent Document 1: International Publication No. 05/065805 pamphlet    Patent Document 2: Japanese Patent Kokai No. 2004-068684    Patent Document 3: Japanese Patent Kokai No. 2000-117049    Patent Document 4: Japanese Patent Kokai No. 2000-051653    Non-patent Document 1: Luke Chen, Jin-Wel Lin and Chen-Lu Yang, “Absorption of NO2 in a Packed Tower with Na2SO3 Aqueous Solution,” Environmental Progress, vol. 21, No. 4, pp. 225-230 (2002)